The disclosure of Japanese Patent Application No. 2000-151637 filed on May 23, 2000, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an apparatus and a method for controlling a belt-type continuously variable transmission, and, more particularly, to a control process performed when a clamping pressure for securing sufficient belt tension is raised to a high level at the time of a failure of a belt clamping pressure control device.
2. Description of Related Art
A belt-type continuously variable transmission that is conventionally employed in a vehicle drive apparatus is disposed in a power transmission path, and includes a pair of pulleys whose groove widths are variable, and a torque transfer belt that is wound around the pulleys for power transmission or torque transfer by use of frictional force. The continuously variable transmission is able to continuously change the speed ratio, which is a ratio of the input-shaft rotation speed on a power source side to the output-shaft rotation speed on the side of drive wheels (i.e., input-shaft rotation speed/output-shaft rotation speed). The belt-type continuously variable transmission usually includes a speed ratio control device that controls the speed ratio of the transmission by changing the groove widths of the pulleys, and a clamping pressure control device that controls the belt clamping pressure for clamping the torque transfer belt. One example of such a belt-type continuously variable transmission is disclosed in Japanese laid-open Patent Publication No. 11-182666. In a vehicle disclosed in this publication, the speed ratio and the belt clamping pressure are controlled by use of hydraulic pressure. When a linear solenoid valve for controlling the belt clamping oil pressure fails, a clutch pressure applied from a forward/reverse-drive switching mechanism is used to control the belt clamping pressure so as to prevent an excessively large load from being imposed on the torque transfer belt or to avoid slippage of the belt.
However, the control of the belt clamping pressure using the clutch pressure of the forward/reverse-drive switching mechanism requires an oil path(s), a changeover or switch valve(s), a solenoid-operated valve(s), etc., for switching oil flow paths in a hydraulic circuit, which makes the apparatus complicated in construction and increased in cost.
It is an object of the invention to provide control apparatus and method of a continuously variable transmission, which employs a simple arrangement for preventing an excessively large load from being applied to a torque transfer belt to reduce the durability thereof, when the clamping pressure is raised to a high level upon a failure of a mechanism that controls the clamping pressure.
To accomplish the above and/or other objects, the invention provides an apparatus for controlling a belt-type continuously variable transmission of a motor vehicle that is provided in a power transmission path, which transmission includes an input shaft disposed on a power source side of the transmission, an output shaft disposed on a side of the transmission opposite from the power source side, a pair of pulleys mounted on the input shaft and the output shaft, respectively, and a torque transfer belt that is wound around the pulleys to effect power transmission by use of frictional force. The transmission has a continuously variable speed ratio, which is a ratio of a rotation speed of the input shaft to a rotation speed of the output shaft. A controller of the apparatus controls the speed ratio of the continuously variable transmission by changing a groove width of each of the pulleys, and controls a belt clamping pressure applied to at least one of the pulleys to clamp the torque transfer belt. The controller also sets the speed ratio of the transmission to a low-load speed ratio when it is determined that control of the clamping pressure has failed and the belt clamping pressure is raised to a high level, the low-load speed ratio being predetermined so that a load that is applied to the torque transfer belt is substantially minimized.
In the control apparatus of the belt-type continuously variable transmission, when the belt clamping pressure is increased at the time when control of the clamping pressure fails, the speed ratio of the transmission is set to the low-load speed ratio that is predetermined so that a stress to be imposed on the torque transfer belt is substantially minimized. Thus, the load applied to the torque transfer belt is reduced, which leads to an improved durability of the belt. The low-load speed ratio may be substantially set to 1.0 if the control range of the speed ratio includes 1.0. In this case, the belt engagement diameter of one of the pulleys is made substantially equal to that of the other pulley. If the control range of the speed ratio does not include 1.0, the low-load speed ratio may be set to a value that is as close to 1.0 as possible. In this connection, if the belt engagement diameter (effective diameter) of the one of the pulleys becomes greater or less than that of the other pulley, the area of contact of the torque transfer belt with the pulley having the smaller belt engagement diameter is reduced, resulting in an increase in the load per unit area. However, if the speed ratio is substantially equal to 1, that is, if the belt engagement diameters of the pair of pulleys are equal to each other, even load is applied to the torque transfer belt, and the maximum load per unit area is reduced.
According to the invention, the control of the belt clamping pressure at the time of a failure is accomplished by keeping the speed ratio of the belt-type continuously variable transmission at the predetermined low-load speed ratio. Accordingly, the controller has a greatly simplified construction and is available at a reduced cost, as compared with the conventional arrangement in which the belt clamping pressure is controlled by using a clutch pressure of a forward/reverse-drive switching device. If the speed ratio of the continuously variable transmission is fixed to 1.0, the vehicle running performance greatly deteriorates because of an incapability of increasing or decreasing the torque by changing the speed ratio. However, the apparatus according to the invention can accomplish an intended purpose in that the vehicle is allowed to limp-home to a repair shop, or the like, rather than being unable to run due to, for example, breakage of the torque transfer belt. 3.
In one preferred embodiment of the invention, the above-indicated controller also restricts an input torque that is transmitted from the power source to the continuously variable transmission, when it is determined that control of the belt clamping pressure has failed and the belt clamping pressure is raised to a high level. This arrangement makes it possible to effectively prevent an excessively large load from being applied to the torque transfer belt, and to avoid slippage of the belt.
In another preferred embodiment of the invention, the controller sets the speed ratio to the low-load speed when the rotation speed of the output shaft is less than a predetermined value, and sets the speed ratio to a high-speed speed ratio that is smaller than the low-load speed ratio when the rotation speed of the output shaft is equal to or greater than the predetermined value. Therefore, when the speed ratio of the continuously variable transmission is changed to the low-load speed ratio upon a failure of a mechanism for controlling the belt clamping pressure, the speed of rotation of the power source is prevented from exceeding a permissible speed (or upper limit) thereof, or an apparatus in which the transmission is mounted is prevented from failing to operate properly due to a rapid change in the speed ratio. For example, if the belt-type continuous variable transmission is shifted down to the low-load speed ratio while the speed ratio of the transmission is smaller than the low-load speed ratio and the output-shaft rotational speed is higher than a predetermined value, the input-shaft rotational speed and the speed of rotation of the power source are increased due to the downshift, giving rise to a possibility that the speed of the power source exceeds a permissible level (or upper limit), resulting in a serious failure. Also, if the belt-type continuous variable transmission of a motor vehicle is shifted down to a great extent against the driver""s intention, large shift shocks may occur due to the inertia of the power source, resulting in deterioration in the riding comfort.
While the control of the speed ratio to the low-load speed ratio is implemented depending upon the output-shaft rotational speed in the above embodiment, the same control may be implemented depending upon the vehicle speed corresponding to the output-shaft rotational speed. In the case where the power source is an internal combustion engine, such as a gasoline engine, the high-speed speed ratio is at least determined so that the engine does not suffer from over-revolution (or over-speed). However, the high-speed speed ratio is desirably set to a speed ratio that is smaller than the speed ratio at which occurrence of over-revolution can be prevented, so as to avoid a situation in which the riding comfort is greatly deteriorated due to a large degree of downshift against the driver""s intention. The above-indicated predetermined value of the output-shaft rotational speed or vehicle speed may be determined so that the riding comfort will not be greatly deteriorated even in the case where the transmission is shifted down from the minimum speed ratio. The high-speed speed ratio may be set to a predetermined fixed value, or may be set to a varying value that depends upon a parameter, such as the output-shaft rotational speed or the vehicle speed. For example, the high-speed speed ratio may be made constant irrespective of the vehicle speed.